Hereinafter, a “Q” prefix in a word of phrase is indicative of a reference of that word or phrase in a quantum computing context unless expressly distinguished where used.
Nature—including molecules—follows the laws of quantum mechanics, a branch of physics that explores how the physical world works at the most fundamental levels. At this level, particles behave in strange ways, taking on more than one state at the same time, and interacting with other particles that are very far away. Quantum computing harnesses these quantum phenomena to process information.
The computers we use today are known as classical computers (also referred to herein as “conventional” computers or conventional nodes, or “CN”). A conventional computer uses a processor fabricated using semiconductor technology, a semiconductor memory, and a magnetic or solid-state storage device, in what is known as a Von Neumann architecture. Particularly, the processors in conventional computers are binary processors, i.e., operating on binary data represented in 1 and 0.
A quantum processor (q-processor) uses the odd nature of entangled qubit devices (compactly referred to herein as “qubit,” plural “qubits) to perform computational tasks. In the particular realms where quantum mechanics operates, particles of matter can exist in multiple states-such as an “on” state, an “off” state, and both “on” and “off” states simultaneously. Where binary computing using semiconductor processors is limited to using just the on and off states (equivalent to 1 and 0 in binary code), a quantum processor harnesses these quantum states of matter to output signals that are usable in data computing.
Conventional computers encode information in bits. Each bit can take the value of 1 or 0. These 1s and 0s act as on/off switches that ultimately drive computer functions. Quantum computers, on the other hand, are based on qubits, which operate according to two key principles of quantum physics: superposition and entanglement. Superposition means that each qubit can represent both a 1 and a 0 at the same time. Entanglement means that qubits in a superposition can be correlated with each other in a non-classical way; that is, the state of one (whether it is a 1 or a 0) can depend on the state of another, and that there is more information that can be ascertained about the two qubits when they are entangled than when they are treated individually. Using these two principles, qubits operate as more sophisticated processors of information, enabling quantum computers to function in ways that allow them to solve difficult problems that are intractable using conventional computers. IBM has successfully constructed and demonstrated the operability of a quantum processor (IBM is a registered trademark of International Business Machines corporation in the United States and in other countries.)
A large roadblock to the development of universal quantum computers is their sensitivity to noise and errors. Theoretically, noise and error sensitivity can be remedied with quantum error correction (QEC) if decoherence and error rates are below a certain threshold. However, in practice, the implementation of a fault tolerant quantum computing architecture is beyond the scope of near-term quantum hardware.
The applicability of near term quantum computers, even before the advent of full fault tolerance, is a problem of great interest. One area of great potential is quantum simulation in which one controllable quantum system is used to study the properties of another quantum system. For example, the application of quantum simulation to electronic structure problems often involves estimating the lowest energy state of a molecular Hamiltonian. Such simulations typically involve the preparation of a quantum state, using a short-depth circuit, and measuring the expectation value of an observable of interest. However, decoherence during state preparation affects the accuracy of the estimated expectation value.